A Schottky emission electron source (hereinafter also briefly referred to as “SE electron source”) is used in scanning electron microscopes (SEM) with a high resolution at low acceleration voltage and electron irradiation apparatuses for element analyses at high speed with X rays, because the SE electron source gives monochromatic, highly bright electron beams and easily provides a large probe current. This electron source includes a single-crystal tungsten (100) rod having a diameter of about 0.1 mm, the tip (apex) of the rod is sharpened, the surface of which is allowed to adsorb zirconium (Zr) and oxygen (O) to selectively allow the (100) plane alone to have a lower work function of about 2.8 eV. The single-crystal tungsten rod is fixed on a heater composed of tungsten filament and is heated to a temperature of about 1800 K upon use.
It has been believed that such customary Schottky electron sources using the ZrO/W system preferably have a large radius r of curvature of the tip of the electron source, in order to obtain a large-current probe and to give electron beams which have a uniform energy width, namely, which are monochromatic. This is disclosed typically in Patent Literature (PTL) 1. PTL 1 mentions that the radius r of curvature is preferably 1 μm or more and 2 μm or less.
Independently, an electron source preferably has such a shape that an electric field focuses on the vicinity of the tip of electron source, and the tip should have a radius r of curvature and a cone angle (opening angle) α within certain ranges, so as to be an electron source with high brightness. This is disclosed typically in PTL 2. PTL 2 mentions that the electron source has a radium r of curvature in the range of from 0.2 μm to 0.5 μm, and a cone angle α in the range of from 5° to 8°.
For obtaining an electron source being more monochromatic and giving a larger current, the electron source should have a surface structure having a work function lower than that of the electron source using Zr—O/W and is capable of operating at low temperatures. The electron source using Zr—O/W has a work function of 2.8 eV and an operating temperature of around 1800 K, whereas the electron source using Ba—O/W has a work function of about 1.0 to about 1.5 eV and an operating temperature of around 1000 to 1200 K and shows an energy width approximately half the energy width of the electron source using Zr—O/W. This is described in Non Patent Literature (NPL) 1 in FIG. 4 and in description from the beginning of Section 6.2 to 4th line therebelow in the right hand of page 441.
The Schottky electron source containing barium has a problem of being unstable in diffusion. To improve this problem, there is disclosed a technique of adding a reductant to a diffusion source provided in the mid-slope of a single-crystal tungsten needle serving as an electron source. This technique is disclosed typically in PTL 3 with candidates of various elements and compounds as materials therefor.